Wide-band coupling circuits



Aug. 12, 1941. v. R. BECK WIDE-BAND coUPLING CIRCUITS Filed AApril 15, v1940 .......MFIIT I Il y, l www IIII IILTIIIIlIl.

Patented Aug. 12, 1941 UNITED STATE WIDE -BAND COUILING CIRCUITS Virgil R. Beck, Chicago, Ill., assigner to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application April 13, 1940, serial No. 329,554

4 claims. k(cl. 25o- 20) This invention relates to radio-frequency coupling circuits, and more particularly to coupling circuits capable of providing an efficient transfer of signal energy over a wide band of radio frequencies.

The coupling circuit provided by the present invention is especially adapted for use in the radio frequency stage 011 stages of radio receivers and the like, and particularly in receivers wherein certain advantages of an R. F. stage are desired, without the disadvantage of having to pro- Qvide an extra tuning condenser section for the tuning of that stage.

It is known :to employ resistance-coupled R. F. stages, or to employ resistance coupling in conjunction with a peaking coil serially connected `between the anode of an R. F. amplifier and the grid of a following tube, to provide increased gain at frequencies where the gain might otherwise be low. Such a coupling circuit is illustrated in the cop-ending application of David Grimes, Serial No. 277,276. iiled June 3, 1939.

The present circuit constitutes an improvement over prior circuits, particularly when employed in receivers of the superheterodyne type, in that the present circuit simultaneously provides efficient signal transfer over the desired band, has a relatively flat gain characteristic, and strongly attenuates image and intermediate frequencies.

Accordingly, lt is an important object of this invention to provide an untuned, wide-band, radio-frequency coupling circuit of simple construction and capable of efficiently transferring signals covering a wide, predetermined range of frequencies.

Another object of this invention is to provide, for a superheterodyne radio receiver or the like, an untuned R. F. stage providing good rejection of image signals, and of signals having frequencies close to the intermediate frequency of the receiver.

The invention may best be understood from the following description in connection with the accompanying drawing, in which Fig. 1 is a schematic diagram of a portion of a radio receiver embodying the present invention, and

Fig. 2 is a more detailed representation of a portion of the circuit of Fig. 1.

The circuit shown in Fig. 1 includes a pair of vacuum tubes V1 and V2, which may function as radio frequency amplifier and detector-oscillator respectively. Connected to the input of the amplifier V1 there may be a suitable source of signals l, such as an antenna, or the output circuit `of a preceding stage. Preferably either the signal source itself or the coupling between the source and the amplifier is tuned, for example, by means of a variable tuning condenser 2 which may be v ganged with the oscillator section 3 as shown.

The invention is particularly directed to the coupling means connected between the output circuit of Vl and the input circuit of V2. In the specific embodiment shown, the coupling network comprises the plate load resistor l; a condenser 5 and inductor 6 serially connected between the plate of V1 and the grid of V2; and the condenser l and inductor serially connected between the input grid and cathode of V2. If desired, a suitable gain control or AVC voltage may be applied to the input grid of V2 by way of the filter resistor 9 and the inductor. 8. The condensers 5 and l are preferably of the trimmer variety, and capable of adjustment over a predetermined, range by means of a screw driver or the like. Preferably also some inductive coupling may be provided between the inductors l and 8, and the elements 5 to 9 inclusive may conveniently be housed in a shield can lil.

The equivalent circuit of the coupling network of Fig. 1 is shown in Fig. 2, wherein C0 and Cl represent the output and input capacities of the tubes V1 and V2 respectively, and C6 and Cs represent the distributed capacity of the inductors 6 and 8 respectively. Mutual inductance between the induotors is indicated by the letter M.

The following is illustrative of one particular application of the circuit to a radio receiver of the superheterodyne type covering the broadcast band from about 550 to 1500 kilocycles and having an intermediate frequency of 455 kilocycles. In such a receiver the trimmer condenser 'l is preferably adjusted to resonate with the inductor 8 at the intermediate frequency, here 455 kilocycles. The elements 1 and 3 thus are enabled, at this frequency, to function as a highly eilicient wave trap to interfering I. F. signals. This is obviously a particularly important consideratio-n where an untuned R, F. stage is employed in place of the more usual tuned stage which would, per se, provide a certain amount of I. F. signal attenuation. The trimmer 5 may be adjusted for maximum overall gain somewhere near, and preferably within, the low frequency limit of the frequency band, for example, 600 kc. Under these conditions, there will be a resonant condition existing between the circuit elements C0, 5, 6 and Cs in parallel, and the seriesparallel group of elements Cl, Cs, 8 and l. Ac-

cordingly at this frequency (600 kc.) the gain of the network will be high notwithstanding the practically negligible gain existing at the intermediate frequency of 455 kc.

At the medium frequencies, and at the high frequency end of the band, overall gain may be maintained at a favorable level by virtue of resonances between the input capacity C1 of V2 and the inductors 6 and 8 together with their distributed capacities Cs and Cs. The mutual coupling M between these inductors, while not crtical, can advantageously be adjusted in any particular case to assist in procuring high frequency resonance at a desired point in the pass band, say at 1400 kc. It has been found that the gain of the network may be made relatively flat between the high and low frequency resonance frequencies, one experimental model exhibiting a maximum gain variation over the band (550- 1500 kc.) approximately in the ratio of 7 to 9. Above 1500 kc., however, the gain falls olf rather rapidly, and as a result the coupling system discriminates very favorably against image signals having frequencies in excess of say 1600 kc.

In general it will be found desirable to keep the value of the plate resistor 4 relatively low in order to maintain the Variation of gain vs. frequency within reasonable limits. In the above-mentioned application of the invention to service in the broadcast band, resistance Values in the order of 5000 and 10000 ohms were found satisfactory. In another model the approximate values of the parts illustrated in Fig. 1 were as follows: Resistor 4-4700 ohms; condensers 5 and '1 -32 and 40 puf. respectively; inductors 6 and 8-1200 and 3060 /Lh respectively, with a mutual inductance therebetween of 335 ch.

It will of course be understood that untuned coupling systems of the type herein disclosed are by no means restricted to use in the broadcast band, and that the Values of the various elements may Vary considerably depending upon the particular band being covered.

I claim:

1. In a superheterodyne radio receiver having a radio frequency amplifier tube and a first detector tube, a source of signals connected to the input circuit of said first-mentioned tube, means for tuning said input circuit over a desired range of frequencies, and an untuned, wide-band 'fr-type coupling network connected between the output circuit of said first-mentioned tube and the input circuit of said second-mentioned tube, said network being capable of efliciently transferring signals within said desired range of frequencies, the input shunt arm of said network comprising an output load resistor for said first-mentioned tube, the series arm of said network comprising a first condenser and first inductor serially connected therein, the output shunt arm of said network comprising a second condenser and a second inductor serially connected therein and adjusted to give series resonance at the intermediate frequency of said receiver, said rst condenser being preadjusted to give maximum overall gain at frequencies near the low frequency end of said desired range of frequencies.

2. A radio receiver according to claim 1, characterized in that said first and second inductors are inductively coupled.

3. In a superheterodyne radio receiver having a radio frequency amplier tube and a rst detector tube, an antenna system connected to the input circuit of said first-mentioned tube, means for tuning said antenna system over a frequency band extending from approximately 550 to 1500 kilocycles, and an untuned coupling network connected between the output circuit of said firstmentioned tube and the input circuit of said second-mentioned tube, said network being capable of efliciently transmitting all signals within said frequency band; said network comprising an output load resistor for said first-mentioned tube, a condenser and inductor serially connected between an output electrode of said first-mentioned tube and an input electrode of said second-mentioned tube, and a second condenser and second inductor serially connected between the input electrodes of said second-mentioned tube and adjusted for series resonance at the intermediate frequency of said receivensaid first-mentioned condenser being preadjusted for maximum network gain at approximately 600 kilocycles.

4. A superheterodyne radio receiver according to claim 1, characterized in that said inductors are coupled inductively, and that high frequency gain is maintained by virtue of resonances between said inductors, their distributed capacity, and the input capacity of said second-mentioned tube, said resonances tending to give high frequency peaking at approximately 1400 kilocycles.

VIRGIL R. BECK. 

